The present invention relates to digital communication receivers, and, more particularly, to a digital communications receiver for recovering data which has been phase-shift modulated onto a communications system carrier waveform.
It is known that digital information may be transmitted in a communications system by phase-shift modulation, wherein the phase of a carrier waveform is inverted, i.e. shifted by 180.degree., during each bit time interval in which a first binary data value is to be transmitted, and is maintained with the same phase as the immediately preceding bit time interval when the remaining binary data value is to be sent in a particular bit time interval. Alternatively, one phase may represent one binary data value and the 180.degree. shifted phase the second binary data value. It is also known that digital information may be transmitted over powerlines between a central facility and a multiplicity of remote locations tied to the power distribution network. Powerline communication systems of this type are disclosed and claimed in Fong--U.S. Pat. No. 3,973,087 and Fong--U.S. Pat. No. 3,973,240, both issued Aug. 3, 1976; Fong--U.S. Pat. No. 3,944,723 issued Mar. 16, 1976; Bogacki et al.--U.S. Pat. No. 4,135,181, issued Jan. 16, 1979; and Bogacki--U.S. Pat. No. 4,161,720, issued July 17, 1979, all assigned to the assignee of the present application and all incorporated herein in their entirety by reference. In powerline communication systems, wherein a powerline frequency (typically 60 Hertz (Hz) in the United States) signal is present throughout the entire system, it is known that the signal-to-noise ratio of the received signal can be greatly improved if the transmitted carrier signal frequencies are odd multiples of the first sub-harmonic, i.e. 30 Hz, of that powerline frequency. Methods and apparatus for realizing such improvements in signal-to-noise are described and claimed in Stutt et al.--U.S. Pat. No. 4,101,834, issued July 18, 1978; and Kincaid et al.--U.S. Pat. No. 4,109,204, issued Aug. 22, 1978, both assigned to the assignee of the present application and incorporated herein by reference in their entirety.
In a data communications system in which the carrier is continuously present, even when the carrier is not modulated to transmit digital data to a receiving location, one presently known receiver for filtering out both systematic pulse and random noise to improve the signal-to-noise ratio is described and claimed in Fong--U.S. Pat. No. 3,944,932, issued Mar. 16, 1976, to the assignee of the present invention and incorporated herein by reference in its entirety. The receiver of U.S. Pat. No. 3,944,932 utilizes at least one narrow band-pass commutating filter preceded by at least one carrier-amplitude-limiting stage. The commutating filter requires a large number of precision resistance and capacitance elements having relatively high cost, and also requires careful adjustment of a number of control elements during assembly, further adding to production cost. A receiver having high rejection of undesirable signals, but also having a relatively low production cost, is highly desirable.
In another known receiver, described and claimed in Hughes--U.S. Pat. No. 4,298,986, issued Nov. 3, 1981, and assigned to the assignee of the present invention and incorporated herein in its entirety by reference, an improved signal-to-noise ratio is achieved, inter alia, by providing a phase-locked loop to generate a loop signal having a frequency essentially locked to the received carrier frequency, whereby synchronous detection is effected. However, little rejection of the harmonics of the powerline frequency, especially those harmonics having an amplitude greater than the level of the desired signal, is provided, thus causing a degradation in performance of the receiver under this condition. Most of the noise in a powerline carrier system is from harmonics of the powerline frequency. Thus it is desirable to have a receiver capable of maintaining harmonic rejection of these frequencies.
Accordingly, an object of the present invention is to provide a receiver having increased signal-to-noise performance when the amplitude of the harmonics of the powerline frequency are greater than the amplitude of the message signal.
Another object of the present invention is to provide a receiver with no narrow bandpass commutating filter and therefore having relatively few precision resistors and capacitors.
Yet another object of the present invention is to provide a receiver having a relatively low cost and no adjustment of control elements.
Still another object of the present invention is to provide a receiver capable of recovering transmitted data modulated onto a carrier signal waveform, wherein the recovered data has the same sense as the transmitted data.